The present invention generally relates to an oxygen sensor that is of the membrane-covered bipolar type, more particularly an oxygen sensor that is suitable for in vivo as well as in vitro measurement of the pO.sub.2 of blood or other body fluids when administering an anaesthetic and to the method of using same. Sensors of this invention are particularly useful when they are designed so as to be incorporated within a catheter whereby the portion of the device for contacting the fluid to be measured may be maneuvered to a desired location within the body of the patient to whom an anaesthetic had been administered. The oxygen sensor is capable of measuring the oxygen content at such a location in a manner that achieves enhanced accuracy by substantially eliminating the interference effects of the anaesthetic with the oxygen measurement mechanism.
Membrane-covered bipolar oxygen sensors, which are generally known as sensors of the Clark cell type, are known to be suitable for the determination of the oxygen content of blood and the like, including in vivo applications by which the cell is inserted into the patient by means of a catheter. Determining the oxygen content of blood or other body fluids is of great importance when, for example, monitoring the condition of a patient who is being treated while being anaesthesized. If the anaesthetic used in this regard is one of the common inhalation anaesthetics such as Halothane (2-bromo-2-chloro-1,1,1-trifluoroethane), Enflurane (2-chloro-1-difluoromethoxy-1,1,2-trifluoroethane), nitrous oxide and the like, such anaesthetics enter the blood stream of the patient and almost inevitably find their way to the location at which the oxygen concentration is being measured by the device, which creates a problem since these anaesthetics typically affect the determination of the oxygen content of the fluid. Oxygen is reduced at the cathode of a Clark-cell type of sensor, and this problem arises primarily because anaesthetics such as Halothane are likewise reduced at the cathode of a Clark-cell type of sensor, the problem being particularly difficult since the electrode potential range of electroreduction of anaesthetics such as Halothane overlaps the electrode potential range of electroreduction of oxygen.
As a result, the current that is measured at a fixed electrode potential by a Clark-cell type of sensor is not defined umambiguously, but the current measured at that fixed electrode potential is the sum of the currents generated by the reduction of oxygen and by the reduction of Halothane or the like. Moreover, this measurement of the oxygen content is further adversely affected by the fact that, due to the low rate of diffusion of anaesthetics such as Halothane through a membrane-covered bipolar oxygen sensor of the Clark cell type, a trailing effect occurs in the sense that a Halothane reduction current is still being measured long after the administration of Halothane to the patient has been terminated.
The present invention substantially eliminates or significantly decreases this type of interference with oxygen concentration measurement that is encountered by a membrane-covered bipolar oxygen sensor, which interference is generated by the presence of an anaesthetic within the body fluid being monitored. This decrease in interference is accomplished primarily by utilizing a cell electrolyte composition that includes a component which exhibits adsorption to the cathode in preference to adsorption of the anaesthetic to the cathode. The invention is based primarily upon the discovery that, for anaesthetics such as Halothane, the first stage in their electroreduction process concerns the adsorption thereof to the cathode surface of a membrane-covered bipolar oxygen sensor of the Clark cell type. This is illustrated by the following reaction equations: ##STR1##
The adsorption illustrated above is substantially prevented in accordance with the present invention. It is suprising that a solution to this problem of anaesthetic interference with oxygen concentration measurement is solved by including a substance within the electrolyte that adsorbs to the cathode and in preference to this adsorption of an anaesthetic such as Halothane to the cathode surface. This preferential adsorption of the electrolyte component results in a change in the properties of the cathode surface, which change typically has an effect only on the activity of the anaesthetic but does not have a substantial effect on the oxygen-reduction process.
It is accordingly a general object of this invention to provide an improved membrane-covered bipolar oxygen sensor of the Clark-cell type.
Another object of the present invention is to provide an improved membrane-covered bipolar oxygen sensor and method of its use that completely or substantially eliminate adverse effects on the measurement of the oxygen content within a body fluid that are caused by the occurrence of a cathodic reduction that fully or partially coincides with that of oxygen.
Another object of the present invention is to provide an improved membrane-covered bipolar oxygen sensor and method which rely upon the adsorption of an electrolyte component to the cathode, which component adsorption is in preference to that of an anaesthetic that may be present within the fluid being measured.
Another object of this invention is to provide an improved membrane-covered bipolar oxygen sensor and method that substantially prevent the reduction of an anaesthetic such as Halothane at the electrode surface while avoiding an significant effect on the reduction of oxygen at the electrode surface.
Another object of the present invention is to provide an improved membrane-covered bipolar oxygen sensor and method that exploit the discovery that the adsorption of an anaesthetic such as Halothane is the first step in the electroreduction process of the anaesthetic.
Another object of the present invention is to provide an improved membrane-covered bipolar oxygen sensor and method that exhibit preventive adsorption of an electrolyte component to its cathode in order to change the properties of the cathode surface without significantly affecting the oxygen reduction process.
These and other objects, features and advantages of this invention will be clearly understood through a consideration of the following detailed description.